BIM-433-40 Radiometrix, BIM-433-40 Datasheet - Page 13

• Manufacturer Part Number: BIM-433-40
• Description: Low Power UHF Data Transceiver Module
• Manufacturer: Radiometrix

Linear operation
A pair of transceivers may also be viewed as a linear analogue channel with a pass baseband of 100Hz
to 17kHz with <10% distortion. The ultimate S/N ratio being >40dB (see quieting curves v RF input).
The test circuit shows the TXD input biased for linear operation and a simple digital filter to shape the
transmit data to a raise cosine wave shape. The 22kΩ resistor linear- biases the TXD input. The drive
voltage should be between 3.5 and 5V pk to pk to achieve full modulation (greatest S/N at receiver)
figure 9: linear drive
Raised cosine shaping may be applied externally to any serial data stream and will yield better error
performance than unshaped data at high data rates (up to 40kbit/s) for data steams with 50:50
mark:space (4ms averaging period). Several excellent modem chips (FX 589 & FX 909) are available for
Consumer Microcircuits Ltd (CML Tel [phone]). These chips employ GMSK (shaped data and
matched receive filters) and enable operation up to 40kbit/s.
figure 10: raised cosine generator
Digitised analogue data
Linear operation of BiM transceivers will allow direct transfer of analogue data, however in many
applications the distortion and low frequency roll off are too high (e.g. bio-medical data such as ECG).
The use of delta modulation is an excellent solution for analogue data in the range 1Hz up to 4KHz with
less than 1% distortion. A number of propitiatory IC's
such as Motorola's MC3517/8 provide CVSD Delta mod/demod on a single chip.
Where the signal bandwidth extends down to DC , such as strain gauges, level sensing, load cells etc.
then V-F / F-V chips (such as Nat Semi LM331) provide a simple means of digitising.
Packet data
Radiometrix Ltd, BiM Data Sheet
page 13